1. Field of the Invention
The present invention relates generally to a method for forming ceramic fabrics by removing a carbon substrate from a fibrous preform and, more particularly, to a method for removing the carbon core from a fiber which has been coated by using a chemical vapor deposition (CVD) or other technique wherein the carbon core is removed from the fiber by oxidation after the fibers have been laid-up to form a preform. Once the fibers have been laid-up to form the preform, the distances between the fiber "ends" generally are small and therefore the cores can be more easily and economically oxidized.
2. Prior Art
The general idea of oxidizing the carbon core of an element produced by CVD is known. Veltri, et al., U.S. Pat. No. 4,500,483, discloses a process for the CVD manufacture of hollow silicon nitride articles by forming a substrate or pattern from carbon, coating the pattern by CVD of silicon nitride, selectively removing a portion of the coating to expose an area of the pattern, and oxidizing the pattern to form a hollow article. The Veltri, et al. process, however, is only applicable to large articles and was not developed or contemplated for fiber substrates on the micrometer level. Hamling, U.S. Pat. No. 3,385,915, discloses a process for producing metal oxide fibers, textiles, and shapes by first impregnating an organic preform with metal salts, and then heating the impregnated organic material to oxidize the carbon and convert the metal salt to a metal oxide. The Hamling process is directed to the fabrication of thin-walled tubes of the material itself and elements having complex shapes from preforms comprising fibers or woven textiles, and not to the removal of a carrying substrate. Pollock, et al., U.S. Pat. No. 4,982,068, discloses a heating element comprising a body of fibrous tubes by coating a carbon fiber with a material, such as SiC, and subsequently removing the carbon fiber core by cutting the coated tubes into appropriate lengths prior to oxidation. The Pollock, et al. process also is directed to large articles, specifically fluid tubes, and not to fabrics comprising hollowed fibers.
Until the present invention, it was not known to oxidize a carbon substrate of a fibrous preform, that is, to remove the substrate after the coated fibers have been weaved into a cloth and laid-up in layers to form the preform. The Pollock, et al. process discloses the removal of the carbon fiber core from a fibrous carbon precursor by oxidation; however, the problems associated with removing the carbon from coated carbon fibers which are relatively long is not confronted because the fibrous carbon precursor is cut into appropriate lengths prior to oxidation. In Pollock, et al., the fibrous carbon precursors range in length from 5 to 1000 millimeters prior to being coated by a plasma activated deposition process (PAVD). Once the fibrous carbon precursor has been coated using the PAVD coating process, the precursor is cut into appropriate lengths for oxidation. According to the examples provided in Pollock, et al., these lengths range from 12.3 millimeters to 15.0 millimeters. Therefore, the carbon comprising the fibrous carbon precursor can be easily oxidized due to the relatively short length of the coated precursor after it has been cut to facilitate oxidation of the carbon.
Many situations arise in which the distances between the ends of the carbon fibers comprising a manufactured article are not small, but are on the order of centimeters in length and, therefore, the oxygen cannot penetrate into the small fiber holes through the entire length of the fibers. Accordingly, a need exists in the industry for a method for removing the carbon substrate from a fibrous preform comprising relatively long lengths of fibers which can be conveniently and economically accomplished without requiting additional processing steps which may damage the fibrous preform.